Disks are a common means for data storage. Data storage is accomplished by altering small areas of the disk surface. The small areas of the disk surface are altered to represent binary ones and zeros.
As is known, read and write heads are typically used for altering the disk surface. Some hard-disk drives use dual element heads. Dual element heads comprise separate reader and writer elements. This allows the writer element to be optimized for writing, and allows the reader element to be optimized for reading. One typical application has an inductive write element and a magnetoresistive read element.
Because the writer and reader elements of a dual element head assembly are at different physical locations, the same information may not be written and read simultaneously with both of those elements. As the reader element is optimized for reading, it is a good choice for reading both data and servo information. As the writer element is optimized for writing, it is a good choice for writing both data and servo information.
However, because dual element heads are at different physical locations, and because different elements are used for reading and writing, a certain amount of offset must be introduced into the servocontrol system. Introducing offset into the servocontrol system provides "trackwise" alignment between read and write elements.
Issues of trackwise alignment for dual element heads are disclosed in: U.S. Pat. No. 5,073,833 (Best et al.); U.S. Pat. No. 4,802,033 (Chi); IBM Technical Disclosure Bulletin, vol. 20, no. 9, February 1978 (McEfee); IBM Technical Disclosure Bulletin, vol. 21, no. 5, October 1978 (Brede); and IBM Technical Disclosure Bulletin, vol. 17, no. 1, June 1974 (Schwarz).
In Best et al., two sets of servo information are generated for each data track on the disk. These sets are radially displaced by a distance equal to the misalignment between the read and write heads. One set is generated with the write head. The other set is generated with the write head, while the read head is used for positioning using the first written set. The sets are then used to position the heads for reading and writing.
In Chi, the trackwise disparity between heads is first measured and then memorized. A self-calibrating algorithmic routine generates a bias to account for the disparity. Chi describes several algorithms for this purpose. In the Disparity Calibration Routine description, a counter is incremented to re-position the head. The signal strength is then detected. Next, the counter is decremented to again re-position the head. The signal strength is then again detected. This is used to determine whether the heads are in alignment.
To write servo information to a disk surface, a servowriter may be used. A servowriter is a machine dedicated to writing servo signals on one or more disks of a disk pack. The servowriter is typically used in the factory. Thus, information regarding servocontrolling the disk drive may be stored on dedicated areas of disk surfaces at the factory. In some dedicated disk drives, one or more heads are reserved for use by the servocontrol system. Other systems use embedded servo signals.
In embedded servo signal drives, servo information is written upon and between data storage tracks on disks of the disk pack. Servo information is typically not written to the same physical location as data in such systems. In embedded servo signal drives, servo information generally comprises a "track number code" and an "error code". The track number code is used to generate a location signal, and the error code is used to generate an error signal proportional to distance and direction from a center of a desired track. The error code is usually valid over only a limited range, such as plus or minus one track front a center line. When used in combination with a track number code, a larger range of positioning information is available. This range varies from application to application, and may include all encoded locations over all surfaces of the disk pack.